This invention relates to a secondary battery protecting module having a protection circuit for protecting a chargeable battery (secondary battery) such as a lithium ion battery and, in particular, to a method of mounting a lead to be connected to a terminal, such as a terminal of a secondary battery, to a printed board.
Among various secondary batteries, in particular, a lithium ion battery is easily damaged by overdischarge and overcharge. It is therefore essential and indispensable to provide a secondary battery protection circuit for detecting an overdischarged state and an overcharged state of the lithium ion battery as a secondary battery to protect the secondary battery from the overdischarged state and the overcharged state. The secondary battery protection circuit comprises an overdischarge preventing mechanism and an overcharge preventing mechanism. For example, the secondary battery protection circuit is disclosed in Japanese Patent (JP-B) No. 2872365 or Japanese Unexamined Patent Application Publication (JP-A) No. 2001-169477. Typically, an IC chip (protection IC) is used as the secondary battery protection circuit.
The above-mentioned secondary battery protection circuit (protection IC) is disposed on a printed board (circuit board) (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2001-268808). On the printed board, a power MOSFET (power IC) serving as a discharge control switch and a charge control switch, and a plurality of electronic components, such as a resistor and a capacitor, are disposed. A combination of the printed board, the secondary battery protection circuit, the power MOSFET, and the electronic components is called a secondary battery protection module.
The secondary battery protection module is electrically connected to terminals of the secondary battery such as the lithium ion battery. A combination of the secondary battery protection module and the secondary battery is called a battery pack.
The secondary battery has an anode terminal and a cathode terminal as the terminals. In order to establish electrical connection with the anode terminal and the cathode terminal of the secondary battery, the secondary battery protection module has a pair of leads on the printed board. For example, each lead comprises a nickel lead. One of the leads which is to be connected to the anode terminal of the secondary battery is called an anode lead. The other lead to be connected to the cathode terminal of the secondary battery is called a cathode lead.
On the other hand, in order to establish electrical connection with a load or a battery charger, the secondary battery protection module has a pair of external connection terminals on the printed board. One of the external connection terminals is a positive terminal. The other external connection terminal is a negative terminal.
Referring to FIG. 1, a related secondary battery protection module 10A will be described. The secondary battery protection module 10A comprises the printed board (circuit board) 11 having a principal surface 11a and a back surface 11b opposite to each other. On the principal surface 11a of the printed board 11, the secondary battery protection circuit (protection IC), the power MOSFET (power IC), the electronic components, and the leads (the anode lead and the cathode lead) 21A (only the anode lead being illustrated in FIG. 1) are disposed. On the other hand, on the back surface 11b of the printed board 11, the external connection terminals (the positive terminal and the negative terminal) and so on are disposed. Between the anode lead 21A and the cathode lead, the secondary battery protection circuit (protection IC) is connected through an internal wiring of the printed board 11. Between the cathode lead and the negative terminal, the power MOSFET (power IC) is connected through another internal wiring of the printed board.
As shown in FIG. 1, each of the leads 21A to be connected to the anode terminal and the cathode terminal of the secondary battery has an L shape. Specifically, the L-shaped lead 21A has a mounted part 21Aa to be mounted (or disposed) on the principal surface 11a of the printed board 11 and a connected part 21Ab to be connected to the terminal (the anode terminal or the cathode terminal) of the secondary battery.
In the secondary battery protection module 10A, the L-shaped lead 21A is mounted on the principal surface 11a of the printed board 11 in the following manner. It is noted here that, on the principal surface 11a of the printed board 11, a land 50 is formed. At first, a flat lead is prepared. The mounted part 21Aa of the flat lead is connected and fixed to the land 50 of the principal surface 11a of the printed board 11 by using a solder 52. Thereafter, the connected part 21Ab of the flat lead is bent by 90° (right angle) with respect to the mounted part 21Aa. Thus, the flat lead is formed into the L-shaped lead 21A.
As described above, in the secondary battery protection module 10A, the lead is mounted on the principal surface 11a of the printed board 11 by connecting and fixing the flat lead to the land 50 of the principal surface 11a of the printed board 11 using the solder 52 and thereafter bending the flat lead by 90° to form the flat lead into the L-shaped lead 21A.
In the above-mentioned lead mounting method, when the flat lead is bent by 90°, a crack may be caused to occur in the solder 52. In order to avoid occurrence of the crack, the flat lead is bent at a position spaced by a predetermined distance from an edge of the land 50 as shown in FIG. 1. Therefore, the mounted part 21Aa has a length Lm′ greater than a length Ll of the land 50 in a bending direction.
However, even if the flat lead is bent at the position spaced from the edge of the land 50, the solder 52 is subjected to a force in a direction of separating the solder 52 from the land 50. As a result, it is difficult to completely avoid occurrence of the crack in the solder 52.